U.S. patent application number 12/776118 was filed with the patent office on 2011-03-10 for grid fault ride-through for current source converter-based wind energy conversion systems.
This patent application is currently assigned to ROCKWELL AUTOMATION TECHNOLOGIES, INC.. Invention is credited to Jingya Dai, Bin Wu, David Dewei Xu, Navid Zargari.
Application Number | 20110057444 12/776118 |
Document ID | / |
Family ID | 43647130 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110057444 |
Kind Code |
A1 |
Dai; Jingya ; et
al. |
March 10, 2011 |
GRID FAULT RIDE-THROUGH FOR CURRENT SOURCE CONVERTER-BASED WIND
ENERGY CONVERSION SYSTEMS
Abstract
Current source converter (CSC) based wind energy power
conversion systems (WECS) and methods are presented in which a
unified DC link current control scheme is employed to facilitate
grid fault ride-through conditions, with a multiple-mode converter
control system that combines the power flow control capabilities of
the generator-side and grid-side converters, in which transitions
between normal operation and fault condition are achieved
automatically by monitoring the grid voltage without the need for,
or with partial additional ride-through components.
Inventors: |
Dai; Jingya; (Toronto,
CA) ; Xu; David Dewei; (Pickering, CA) ; Wu;
Bin; (Toronto, CA) ; Zargari; Navid;
(Cambridge, CA) |
Assignee: |
ROCKWELL AUTOMATION TECHNOLOGIES,
INC.
Mayfield Hts.
OH
|
Family ID: |
43647130 |
Appl. No.: |
12/776118 |
Filed: |
May 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61239949 |
Sep 4, 2009 |
|
|
|
Current U.S.
Class: |
290/44 ;
700/287 |
Current CPC
Class: |
H02J 3/381 20130101;
H02J 2300/28 20200101; Y02E 10/76 20130101; H02M 5/4505 20130101;
F05B 2270/10711 20130101; H02J 3/386 20130101 |
Class at
Publication: |
290/44 ;
700/287 |
International
Class: |
H02P 9/04 20060101
H02P009/04; G06F 1/26 20060101 G06F001/26 |
Claims
1. A current source converter (CSC) for converting input electrical
power to output electrical power, comprising: an input for
receiving input electrical power from a power source; an output for
providing output electrical power to a power grid; an intermediate
circuit with at least one inductance; a current source rectifier
(CSR) comprising a plurality of switching devices coupled with the
input and with the intermediate circuit and operative according to
a plurality of CSR switching control signals to selectively couple
the input to the intermediate circuit; a current source inverter
(CSI) comprising a plurality of switching devices coupled with the
intermediate circuit and with the output and operative according to
a plurality of CSI switching control signals to selectively couple
the intermediate circuit to the output; and a switch control system
comprising: a generator side control component operatively coupled
with the CSR to provide CSR switching control signals to the CSR to
cause the CSR to convert input power from the power source to
create a DC link current in the intermediate circuit, a grid side
control component operatively coupled with the CSI to provide CSI
switching control signals to the CSI to convert the DC link current
to selectively provide the output electrical power to the power
grid, and a DC link current control component operatively coupled
with the generator side control component and operative in a first
mode to cause the generator side control component to provide the
CSR switching control signals to at least partially regulate the DC
link current.
2. The CSC of claim 1, wherein the DC link current control
component is operatively coupled with the generator side control
component and with the grid side control component, and wherein the
DC link current control component is operative in the first mode to
cause the generator side control component and the grid side
control component to provide the CSR and CSI switching control
signals to jointly regulate the DC link current.
3. The CSC of claim 2, wherein the DC link current control
component is operative in a second mode to cause the grid side
control component to provide the CSI switching control signals to
regulate the DC link current.
4. The CSC of claim 3, further comprising a feedback system
operatively coupled with the intermediate circuit and providing at
least one feedback value or signal to the switch control system
indicative of a voltage across the at least one inductance, wherein
the DC link current control component is operative to selectively
enter one of the first and second modes based at least in part on
the at least one feedback value or signal.
5. The CSC of claim 4, wherein the DC link current control
component is operative to selectively enter the first mode when the
voltage across the at least one inductance is greater than a
maximum DC link voltage control range of the grid side control
component, and to enter the second mode when the voltage across the
at least one inductance is less than or equal to the maximum DC
link voltage control range of the grid side control component.
6. The CSC of claim 3, wherein the DC link current control
component is operative in the second mode to cause the grid side
control component to exclusively regulate the DC link current.
7. The CSC of claim 6, further comprising a feedback system
operatively coupled with the intermediate circuit and providing at
least one feedback value or signal to the switch control system
indicative of a voltage in the intermediate circuit, wherein the DC
link current control component is operative to selectively enter
one of the first and second modes based at least in part on the at
least one feedback value or signal.
8. The CSC of claim 7, wherein the DC link current control
component is operative to selectively enter the first mode when a
voltage across the at least one inductance is greater than a
maximum DC link voltage control range of the grid side control
component, and to enter the second mode when the voltage across the
at least one inductance is less than or equal to the maximum DC
link voltage control range of the grid side control component.
9. The CSC of claim 1, wherein the DC link current control
component is operative in a second mode to cause the grid side
control component to provide the CSI switching control signals to
regulate the DC link current.
10. The CSC of claim 9, further comprising a feedback system
operatively coupled with the intermediate circuit and providing at
least one feedback value or signal to the switch control system
indicative of a voltage across the at least one inductance, wherein
the DC link current control component is operative to selectively
enter one of the first and second modes based at least in part on
the at least one feedback value or signal.
11. The CSC of claim 10, wherein the DC link current control
component is operative to selectively enter the first mode when the
voltage across the at least one inductance is greater than a
maximum DC link voltage control range of the grid side control
component, and to enter the second mode when the voltage across the
at least one inductance is less than or equal to the maximum DC
link voltage control range of the grid side control component.
12. The CSC of claim 9, wherein the DC link current control
component is operative in the second mode to cause the grid side
control component to exclusively regulate the DC link current.
13. The CSC of claim 1, further comprising a feedback system
operatively coupled with the intermediate circuit and providing at
least one feedback value or signal to the switch control system
indicative of a voltage across the at least one inductance, wherein
the DC link current control component is operative to selectively
enter the first mode based at least in part on the at least one
feedback value or signal.
14. The CSC of claim 13, wherein the DC link current control
component is operative to selectively enter the first mode when the
voltage across the at least one inductance is greater than a
maximum DC link voltage control range of the grid side control
component.
15. The CSC of claim 1, wherein the DC link current control
component is operative in a second mode to cause the grid side
control component to exclusively regulate the DC link current.
16. A wind energy conversion system (WECS) for providing electrical
power to a grid, comprising: a generator with a rotor adapted to be
driven by a prime mover and a generator output providing multiphase
electrical output power when the rotor is driven; and a current
source converter (CSC) for converting input electrical power to
output electrical power, comprising: an input for receiving input
electrical power from the synchronous generator; an output for
providing output electrical power to a power grid; an intermediate
circuit with at least one inductance; a current source rectifier
(CSR) comprising a plurality of switching devices coupled with the
input and with the intermediate circuit and operative according to
a plurality of CSR switching control signals to selectively couple
the input to the intermediate circuit; a current source inverter
(CSI) comprising a plurality of switching devices coupled with the
intermediate circuit and with the output and operative according to
a plurality of CSI switching control signals to selectively couple
the intermediate circuit to the output; and a switch control system
comprising: a generator side control component operatively coupled
with the CSR to provide CSR switching control signals to the CSR to
cause the CSR to convert input power from the generator to create a
DC link current in the intermediate circuit, a grid side control
component operatively coupled with the CSI to provide CSI switching
control signals to the CSI to convert the DC link current to
selectively provide the output electrical power to the power grid,
and a DC link current control component operatively coupled with
the generator side control component and operative in a first mode
to cause the generator side control component to provide the CSR
switching control signals to at least partially regulate the DC
link current.
17. The WECS of claim 16, wherein the DC link current control
component is operatively coupled with the generator side control
component and with the grid side control component, and wherein the
DC link current control component is operative in the first mode to
cause the generator side control component and the grid side
control component to provide the CSR and CSI switching control
signals to jointly regulate the DC link current.
18. The WECS of claim 16, wherein the DC link current control
component is operative in a second mode to cause the grid side
control component to provide the CSI switching control signals to
regulate the DC link current.
19. The WECS of claim 16, wherein the CSC further comprises a
feedback system operatively coupled with the intermediate circuit
and providing at least one feedback value or signal to the switch
control system indicative of a voltage in the intermediate circuit,
wherein the DC link current control component is operative to
selectively enter the first mode based at least in part on the at
least one feedback value or signal.
20. The WECS of claim 19, wherein the DC link current control
component is operative to selectively enter the first mode when the
voltage across the at least one inductance is greater than a
maximum DC link voltage control range of the grid side control
component.
21. The WECS of claim 16, wherein the DC link current control
component is operative in the second mode to cause the grid side
control component to exclusively regulate the DC link current.
22. A method for operating a current source converter (CSC) to
convert input electrical power to output electrical power in a wind
energy conversion system (WECS), the method comprising: receiving
input electrical power from a generator with a rotor coupled
directly or indirectly to a wind-driven prime mover at an input of
a current source converter (CSC), the CSC having an intermediate
circuit with at least one inductance; selectively coupling the
input to the intermediate circuit using a current source rectifier
(CSR) of the CSC to convert input power from the generator to
create a DC link current in the intermediate circuit; selectively
coupling the intermediate circuit to the output using a current
source inverter (CSI) of the CSC to convert the DC link current to
provide output electrical power to a power grid; and operating the
CSR in a first mode to at least partially regulate the DC link
current.
23. The method of claim 22, comprising operating the CSR and the
CSI to jointly regulate the DC link current in the first mode.
24. The method of claim 22, comprising operating the CSI in a
second mode to regulate the DC link current.
25. The method of claim 22, comprising selectively entering the
first mode based at least in part on at least one feedback value or
signal from the intermediate circuit.
26. The method of claim 25, comprising selectively entering the
first mode when a voltage across the at least one inductance is
greater than a maximum DC link voltage control range of the CSI.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of U.S.
Provisional Patent Application Ser. No. 61/239,947, filed Sep. 4,
2009, entitled "GRID FAULT RIDE-THROUGH FOR CURRENT SOURCE
CONVERTER-BASED WIND ENERGY CONVERSION SYSTEMS", the entirety of
which is hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to electrical power
conversion and more particularly to techniques for enhanced grid
fault ride-through capabilities of current source converter (CSC)
based wind energy conversion systems (WECS).
BACKGROUND OF THE INVENTION
[0003] Wind energy is currently a fast growing power generation
technology, and ongoing development is directed to providing
wind-generated power to electrical power grids. Power conversion
systems are needed to adapt the power generated by the wind
turbines to AC electric power in a form compatible with the power
grid. One type of conversion apparatus is a current source
converter (CSC) that includes a current source rectifier (CSR) and
a current source inverter (CSI).
[0004] As WECSs become more prevalent, utility operators must
ensure the reliability and efficiency of the power system,
including compliance with grid connection codes applicable to
distributed generators including wind power generators. One such
requirement is the capability of WECSs to ride-through grid fault
conditions to prevent disconnection of a large wind generator
caused by network disturbances to avoid or mitigate system
instability and generator trips. Other typical requirements such as
reactive and active power regulation based on the system voltage
and frequency are also specified. Compliance with these
requirements impacts the design of power converters and controllers
for WECSs. Currently, the most prevalent WECS configuration is a
variable-speed wind turbine used with either a doubly fed induction
generator (DFIG) with a partial-rated power converter or an
induction/synchronous machine equipped with a fully-rated power
converter. DFIG configurations are popular because of the reduced
size of the converter (about 1/3 to 1/4 of the total KVA rating),
but the fault ride-through capability of DFIG systems is limited
and additional hardware is required in most cases. Direct-drive
permanent magnet synchronous generator (PMSG) solutions with a full
power converter are an attractive alternative as these are
completely decoupled from the power grid, and provide wide
operating range with fault-ride-through capability. In addition,
the provision of a permanent magnet rotor (without electrically
excited rotor windings) improves system efficiency and eliminates
the need for slip-ring and maintenance, making the PMSG solution
ideal for high power offshore applications.
[0005] Most conventional drive system control schemes assume static
grid behavior and are thus not well adapted for accommodating grid
fault conditions. A short circuit grid fault and the resultant
converter terminal voltage drop may cause the grid side converter
to lose its control capability. Unbalanced power flow at the input
and output during transients can cause over-current or over-voltage
in the converters and trigger the system protection and ultimately
converter shut down. Previous fault ride-through techniques have
largely been focused on voltage source converters (VSC) in WECS,
such as electronic dynamic braking to dump the excessive energy to
external resistors or energy storage systems, or allowing the
incoming wind energy to be temporarily stored in the moment of
inertia of the turbine-generation system. Other proposed
fault-condition techniques employ nonlinear control methods to
improve the conventional current control performance, but the
implementations are complex and very sensitive to system variables.
Pulse width modulated (PWM) current source converter (CSC)
topologies, compared with VSC based configurations, provide a
simple topology solution and excellent grid integration performance
such as sinusoidal current and fully controlled power factor, where
a DC link reactor provides natural protection against converter
short circuit faults. However, unlike VSCs, the grid voltage fault
ride through of a CSC-based WECS has been rarely studied in the
literature. Accordingly, there is a need for improved wind energy
systems by which energy derived from wind-driven machines can be
converted in a CSC for supplying electrical power to a grid with
the capability of riding through grid fault conditions without
introduction of additional hardware.
SUMMARY OF INVENTION
[0006] Various aspects of the present invention are now summarized
to facilitate a basic understanding of the invention, wherein this
summary is not an extensive overview of the invention, and is
intended neither to identify certain elements of the invention, nor
to delineate the scope thereof. Rather, the primary purpose of this
summary is to present some concepts of the invention in a
simplified form prior to the more detailed description that is
presented hereinafter.
[0007] The present disclosure presents power conversion systems and
current source converters and switching controls thereof by which
CSC-based wind energy and other systems may successfully provide
reactive power control and grid fault tolerance while employing
current source converter technology and the associated advantages.
The disclosed concepts provide novel integrated solutions for
control of CSC-based WECSs in normal operation and/or to
ride-through grid low voltage faults in which generator-side
converter control is used to at least partially regulate the DC
link current in an intermediate DC link circuit of the CSC-based
converter. In this manner, the ability to regulate the DC link
current is extended beyond the capabilities of the grid-side
converter, which is particularly useful in certain grid fault
ride-through scenarios.
[0008] Wind energy conversion systems and current source converters
therefor are provided for converting input electrical power from a
wind-driven synchronous generator and for providing output
electrical power to a power grid in accordance with one or more
aspects of the present disclosure. The CSC includes an intermediate
circuit with at least one link inductance, as well as a current
source rectifier (CSR) with switches operated by CSR switching
control signals to selectively couple the input to the intermediate
circuit, and a current source inverter (CSI) with switches operated
via CSI switching control signals to selectively couple the
intermediate circuit to the output. A switch control system is
provided, having generator and grid side control components, with
the generator side control component providing signals to the CSR
to convert input power from the generator to create a DC link
current in the intermediate circuit, and the grid side control
component providing signals to the CSI to convert the DC link
current to output electrical power for the grid.
[0009] The switching control system also includes a DC link current
control component operative in a first mode to cause the generator
side control component to provide the CSR switching control signals
to wholly or partially regulate the DC link current. In certain
embodiments, unified DC link current control is provided in the
first mode with the DC link current control component causing the
generator side control component and the grid side control
component to jointly regulate the DC link current. In a second mode
in some embodiments, grid side control is used exclusively for
regulating the link current, with the DC link current control
component operating mode being determined according to one or more
feedback signals. In certain embodiments, the DC link current
control component implements unified link current control in the
first mode when the voltage across the link inductance is greater
than a maximum DC link voltage control range of the grid side
converter (CSI), and otherwise grid side control is used to
regulate the link current in the second mode. In this manner, the
DC link current can continue to be regulated at a value necessary
to support grid voltage recovery in grid fault situations where the
grid-side (CSI) control by itself cannot maintain the required DC
link current.
[0010] In accordance with further aspects of the disclosure, a
method is provided for operating a current source converter (CSC)
to convert input electrical power to output electrical power in a
wind energy conversion system (WECS). The method includes receiving
input electrical power in a CSC from a synchronous generator with a
rotor coupled directly or indirectly to a wind-driven prime mover
and selectively coupling the input to an intermediate circuit using
a current source rectifier to convert input power from the
generator to create a DC link current in the intermediate circuit.
The method also includes selectively coupling the intermediate
circuit to the output using a current source inverter to convert
the DC link current to provide output electrical power to a power
grid, and selectively operating the CSR in a first mode to at least
partially regulate the DC link current. Certain embodiments of the
method include operating the CSR and the CSI to jointly regulate
the DC link current in the first mode. In various embodiments, the
method further includes operating the CSI to regulate the link
current in a second mode. In certain embodiments, the first mode is
entered based at least partially on one or more feedback signals or
values from the intermediate circuit, such as when a voltage across
the a link inductance is above the maximum DC link voltage control
range of the CSI.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The following description and drawings set forth certain
illustrative implementations of the disclosure in detail, which are
indicative of several exemplary ways in which the various
principles of the disclosure may be carried out. The illustrated
examples, however, are not exhaustive of the many possible
embodiments of the disclosure. Other objects, advantages and novel
features of the invention will be set forth in the following
detailed description when considered in conjunction with the
drawings, in which:
[0012] FIG. 1 is a schematic diagram illustrating an exemplary
current source converter (CSC) based wind energy power conversion
system (WECS) with a synchronous generator and a switch control
system with a unified DC link current control component for
grid-fault ride-through in accordance with one or more aspects of
the present disclosure;
[0013] FIGS. 2 and 3 are schematic diagrams illustrating further
details of the switch control system in the WECS of FIG. 1;
[0014] FIG. 4 is a graph showing preferred low grid voltage ride
through operation of the WECS of FIGS. 1-3;
[0015] FIG. 5 is a graph showing preferred reactive current control
performance of the WECS of FIGS. 1-3 to support voltage
recovery;
[0016] FIG. 6 illustrates graphs of maximum allowable active
current, required reactive current compensation, and minimum DC
link current to support grid voltage recovery in the WECS of FIGS.
1-3; and
[0017] FIG. 7 is a flow diagram illustrating an exemplary method
for controlling a CSC-based WECS in accordance with further aspects
of the disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Referring now to the figures, several embodiments or
implementations of the present invention are hereinafter described
in conjunction with the drawings, wherein like reference numerals
are used to refer to like elements throughout, and wherein the
various features are not necessarily drawn to scale.
[0019] A unified DC link current control scheme is described to
facilitate ride through of grid faults in CSC-WECS. Possible
implementations assist grid-side active/reactive current regulation
to satisfy grid code requirements, and may be fully integrated into
the switching control system of the CSC with partial or without
additional components for grid fault ride-through capabilities. The
coordinated control of the input and output power can also be
utilized to smooth power output while maintaining fast control
response of the DC link current.
[0020] Referring initially to FIGS. 1-3, FIGS. 1 and 2 illustrate
an exemplary wind energy converter (WEC) or wind energy system
(WES) 100 in accordance with the present disclosure, in which a
current source converter (CSC) 110 is connected to an AC power
source derived from a wind-receiving rotary propeller 112
operatively coupled with a source of electrical power 116, in one
example, a permanent magnet synchronous generator (PMSG) 116
operating in this case as a generator with a permanent magnet
rotor. In other embodiments, the power source 116 can be induction
machine or other type of generator. The system 100 may optionally
include a gearbox 114 operatively coupled between the propeller 112
and the PMSG 116, although not a strict requirement of the present
disclosure. The PMSG 116 converts rotational mechanical energy from
the propeller 112 into single or multi-phase AC electrical power,
which is provided as a machine-side or generator-side power input
to the CSC 110, and the CSC 110 provides a grid-side power output
in the form of multi-phase AC electrical power to a grid 120.
[0021] The CSC 110 converts input (machine-side) power to output
(grid-side) power, and includes a generator-side converter or
current source rectifier (CSR) 110a that converts the input AC
electrical power to DC to feed a DC link intermediate circuit 150
with at least one storage element, such as a DC choke L.sub.dc in
one example. A grid-side converter or current source inverter (CSI)
110b generates the AC power output to the grid 120 derived from the
current I.sub.dc in the intermediate circuit 150. As best shown in
FIG. 1, certain embodiments may include an optional step-up
transformer 115, for example, to step the output voltage (e.g.,
about 6 KV in one example) up to about 30 KV for the grid 120, and
also to provide isolation between the converter 110 and the grid
load 120. The CSR 110a and the CSI 110b are switch-based converters
including electrical switching devices S1-S6 and S7-S12,
respectively, which can be any suitable form of electrical
switches, including without limitation IGCTs, GTOs, SGCTs, IGBTs,
FETs, etc. The exemplary CSC 110, moreover, includes input line
filter capacitors C.sub.r wye-coupled or delta-coupled to the input
nodes A, B, and C in the illustrated embodiment. In addition, the
exemplary drive 110 may also include output grid capacitors C.sub.i
wye-connected or delta-connected to the output lines U, V, and W,
although not a requirement of the present disclosure.
[0022] Although illustrated in the context of a three-phase
electrical conversion system 110, the various power conversion
aspects of the present disclosure may be advantageously employed in
association with single-phase implementations, as well as
multi-phase systems having three or more power lines as input (from
a PMSG) and/or output (to a grid). Moreover, the converter 110 may
be employed in connection with other forms of input generators 116
and is not limited to permanent magnet synchronous type generators
116. The system 100 and the converter 110 thereof, moreover, may be
operated at any suitable input and output frequencies, for example,
wherein the frequency of the input power received from the PMSG 116
may vary with the speed of the wind and the converter 110 is
adaptable to provide AC electrical power of any desired output
frequency to the grid 120. In addition, while illustrated and
described in the context of a wind energy system 100, various
aspects of the present disclosure can be implemented in association
with other forms of CSC-type converters.
[0023] In the illustrated three-phase embodiment of FIG. 1, AC
input power from the generator 116 is switched by a first set of
switches S1-S6 constituting the generator-side converter 110a in
the form of a switching current source rectifier (CSR) to create an
intermediate DC bus current I.sub.ds in the intermediate circuit
150. The grid-side current source inverter (CSI) 110b includes a
second set of switches S7-S12 forming a switching inverter stage
that selectively switches the DC power from the intermediate
circuit 150 to provide multiphase AC output power to the grid 120.
The intermediate circuit 150 includes a DC choke or inductor
L.sub.dc linking the switches of the CSR 110a and the CSI 110b and
provides forward and reverse current paths between the converters
110a and 110b. The inductor L.sub.dc of the exemplary intermediate
circuit 150 includes a first winding WA in a forward or positive DC
path between the upper CSR switches S1-S3 and the upper CSI
switches S7-S9. Other possible implementations may include further
windings, such as a second winding (not shown) in a negative or
return DC path between the lower CSR switches S4-S6 and the lower
CSI switches S10-S12. The switching devices S1-S6 and S7-S12 may be
any suitable controllable electrical switch types (e.g., IGCTs,
SGCTs, GTOs, thyristors, IGBTs, etc.) that are controlled according
to any suitable type or form of switching scheme or schemes, such
as phase control, pulse width modulation, etc., in open or
closed-loop fashion.
[0024] The converters 110a and 110b operate under control of a
switch control system 140 for conversion of input wind power to
grid power, although separate switching control systems may be
employed, for example, with interconnections and information
sharing to facilitate the coordinated operation of the CSR 110a and
the CSI 110b. The illustrated CSC 110 also includes a fault mode
control 160 for modifying the operation of the converter 110 during
fault conditions on the grid 120. CSR switching control signals
142a are provided to the individual switches S1-S6 and CSI signals
142b are provided to the CSI switches S7-S12 from the switch
control system 140 in order to implement a given power conversion
task. The switch control system 140 may be provided with one or
more setpoint desired values and one or more feedback signals or
values from a feedback system 118 by which one or more closed loop
power conversion goals are achieved in normal operation, and by
which the CSC 110 can facilitate operation during grid faults when
the grid voltage(s) drops below a predetermined threshold value. In
the illustrated embodiments, for example, the switch control system
140 provides inputs for receiving a fault mode signal 160, feedback
signals or values from the output feedback system 118, measured
input values (e.g., line voltages, currents, etc.), and other
information, data, etc., which may be in any suitable form such as
an electrical signal, digital data, etc., and which may be received
from any suitable source, such as an external network, switches, a
user interface associated with the system 100, or other suitable
source(s). The switch control system 140 and the components thereof
may be any suitable hardware, processor-executed software,
processor-executed firmware, logic, or combinations thereof that
are adapted to implement the functions illustrated and described
herein.
[0025] In normal operation, the switching devices S1-S6 of the CSR
110a selectively coupled individual ones of the input terminals A,
B, and/or C with the intermediate circuit 150 according to a
plurality of CSR switching control signals 142a so as to convert
input multiphase electric power to DC current I.sub.ds in the
intermediate circuit 150, and the CSI switches S7-S12 are operated
according to the CSI switching control signals 142b to selectively
couple the intermediate circuit 150 to the output so as to provide
multiphase output power to the grid 120. The feedback system 118
provides one or more feedback values or signals to the control
system 140 that are indicative of one or more electrical conditions
at the output, or of the converters 110 or in the intermediate
circuit 150, such as the voltage across the link inductance
L.sub.dc.
[0026] As shown in FIGS. 1 and 2, the switching control system 140
includes a generator-side (CSR) control component 144a providing
the switching control signals 142a to the CSR 110a, a grid-side
(CSI) control component 144b providing the CSI switching control
signals 142b to the CSI 110b. The exemplary system 140, moreover,
includes a wind turbine control component 144c and a grid
supervisory control component 144d, as well as a unified DC link
current control component 144e, which may internally switch between
first and second operational modes based on one or more
signals/values from the feedback system 118 and/or which may modify
its link current regulation technique based in whole or in part on
a fault mode signal 160.
[0027] The DC link current control component 144e in one embodiment
operates in first and second modes, including a first mode where
link controller 144e causes the generator side control component
144a to provide the CSR switching control signals 142a to at least
partially regulate the DC link current I.sub.dc. In certain
embodiments, the component 144e implements unified DC link current
control in the first mode with the generator side control component
144a and the grid side control component 144b jointly regulating
the DC link current I.sub.dc. The DC link current control component
144e operates in a second mode to cause the grid side control
component 144b to regulate I.sub.dc via the CSI switch control
signals 142b. In the CSC 110, the operational mode of the DC link
current control component 144e is set based on one or more signals
from the feedback system 118 to advantageously allow adaptation of
the link current regulation for changing conditions, such as grid
faults.
[0028] In certain embodiments, the DC link current control
component 144e implements unified link current control in the first
mode when the magnitude of the voltage -V.sub.Ldc across the link
inductance L.sub.dc is greater than the maximum DC link voltage
control range V.sub.dci,max of the grid side converter (CSI), and
otherwise (-V.sub.Ldc.ltoreq.V.sub.dci,max) in the second mode the
grid side control 144b is used exclusively to regulate the link
current I.sub.dc. For example, the unified DC link current control
aspects of the switch control system 140 advantageously continues
to regulate I.sub.dc at a value necessary to support grid voltage
recovery in grid fault situations where the grid-side (CSI) control
144b by itself cannot maintain the required DC link current level
I.sub.dc. In particular, the unified control 144e allows the CSC
110 to provide reactive current to support grid voltage recovery
during grid voltage dips and short circuit faults. The control
component 144e accommodates the changed DC link current reference
value I*.sub.dc in such situations, and selectively enters the
first mode to supplement the grid-side link current regulation with
regulation by the generator-side controller 144a to meet the active
and reactive current references required by the needs of the grid
120.
[0029] Referring also to FIG. 7, the operation of the CSC 110 is
further detailed in an exemplary method 300 for operating a current
source converter CSC 110 to convert input electrical power to
output electrical power in a wind energy conversion system such as
the WECS 100 described herein. Although the exemplary method 300 is
depicted and described in the form of a series of acts or events,
it will be appreciated that the various methods of the disclosure
are not limited by the illustrated ordering of such acts or events
except as specifically set forth herein. In this regard, except as
specifically provided hereinafter, some acts or events may occur in
different order and/or concurrently with other acts or events apart
from those illustrated and described herein, and not all
illustrated steps may be required to implement a process or method
in accordance with the present disclosure. The illustrated methods
may be implemented in hardware, processor-executed software, or
combinations thereof, in order to provide CSC-based energy
conversion control functionality including regulation of
intermediate DC link current in WECS control systems such as those
illustrated and described herein, although the invention is not
limited to the specifically illustrated or described applications
and systems. For instance, the method 300 and the CSC concepts
described herein can also be combined with one or more partial
energy storage components, including without limitation braking
resistors, capacitors, and/or batteries.
[0030] In normal operation at 310 in FIG. 7, the method 300
provides for conversion of wind energy to grid power through
receipt of input electrical power from a synchronous generator
(e.g., generator 116 above) rotated by a wind-driven prime mover
(111) at an input of a current source converter (CSC 110), and
selective coupling of the input to an intermediate circuit (150)
using a current source rectifier (e.g., CSR 110a) to convert input
power from the generator to create a DC link current (I.sub.dc) in
the intermediate circuit. The normal operation 310 also includes
selectively coupling the intermediate circuit to the output using a
current source inverter (e.g., CSI 110b of the CSC 110) to convert
the DC link current (I.sub.dc) to provide output electrical power
to a power grid (120), with the DC link current being regulated at
312 using the grid side converter (CSI 110b) through provision of
the control signals 142b by the controller 144b. The DC link
voltage (e.g., V.sub.Ldc in FIG. 1 above) or other feedback
signal/value associated with the intermediate circuit is detected
or estimated at 314. A determination is made at 316 as to whether
the link voltage exceeds the control range of the grid side
converter (e.g., whether -V.sub.Ldc>V.sub.dci,max). If not (NO
at 316), the control remains in the normal operating mode at 310
with the grid side controller 144b solely responsible for
regulation of the DC link current (I.sub.dc).
[0031] If, however, the grid side control can no longer accommodate
the required link current value (YES at 316 where
-V.sub.Ldc.ltoreq.V.sub.dci,max, such as during grid fault ride
through situations), the switching control system (140) enters a
unified control mode (first mode described above) in which unified
control is implemented (via the unified DC link controller 144e in
FIGS. 1-3 above) with the generator side converter (CSR 110a) at
least partially regulating the DC link current I.sub.dc. In one
implementation, the DC link current I.sub.dc is jointly regulated
at 322 in FIG. 7 by both the grid side converter (CSI 110b) and the
generator side converter (CSR 110a) in the first mode at 320. The
DC link voltage (V.sub.Ldc in one example) is again detected or
estimated at 324 and a determination is made at 326 as to whether
this has fallen to or below the control range of the grid side
converter (e.g., whether -V.sub.Ldc.ltoreq.V.sub.dci,max). If so
(YES at 326), the control remains in the unified regulation mode at
320 as described above, and if not (NO at 326), the control system
140 returns to normal mode at 310 with the grid side controller
144b exclusively regulating the DC link current (I.sub.dc).
[0032] As shown in FIG. 1, the switching control system 140 for the
whole WECS 100 includes the wind turbine controller 144c, the
controllers 144a and 144b for generator-side/grid-side converters,
and the grid integration supervisory system 144d. The turbine
controller 144c in certain embodiments measures wind speed and
provides references for turbine pitch control, if available, as
well as a generator speed controller to achieve maximum power point
tracking and proper turbine protection. The grid integration
supervisory controller 144d in certain embodiments monitors the
grid voltage V.sub.s and frequency .omega..sub.s to detect possible
grid operating conditions, such as loss of load or any grid faults.
Based on the detected information and the grid code requirements,
it will issue corresponding commands to the converter control
components of the system 140.
[0033] FIGS. 2 and 3 further illustrate the controllers 144a, 144b,
and 144e in the exemplary switch control system 140, in which field
oriented control (FOC) is developed at the generator side
controller 144a and voltage oriented control (VOC) is employed for
the grid side control 144b in one embodiment, in which `d` and `q`
subscriptions denote the d-axis and q-axis of the selected
synchronous frame, respectively. As shown in FIG. 3, the
generator-side controller 144a provides a MPPT control component
144a2 setting the generator q-axis current i*.sub.qg and an
optimized generator operation component 144a3 that sets the grid
d-axis current *i.sub.dg, and these values are used by a converter
current calculation and gating generation component 144a1 to
generate the switching control signals 142a. In addition, the
grid-side controller 144b includes a DC link current control
component 144b2 setting the d-axis current i*.sub.ds and a reactive
power controller 144b3 that sets the current i*.sub.qs, and these
values are used by a converter current calculation and gating
generation component 144b1 to generate the switching control
signals 142b, where i*.sub.qg and *i.sub.ds are generally related
to the real power control of the system 100. The converter gating
generation can be, for example, space vector modulation (SVM) or
any modulation scheme that can control the amplitude and delay
angle of the reference vector.
[0034] The power flow can be maintained by controlling the
generator speed .omega..sub.g to trace the reference speed from
wind turbine controller 144c, while regulating the DC link current
level I.sub.dc by the grid-side converter 110b to ensure the
balanced power flow at both sides. The reactive power control at
the generator side 144a helps obtain desired generator terminal
voltage V.sub.g and current i.sub.g, minimizing generator line
current or limiting generator terminal voltage. At the grid side
converter 110b, reactive power control is used to regulate the grid
voltage V.sub.dci or to meet other grid operating requirements
(V.sub.dci can be regulated by grid-side reactive power control).
In order to achieve all these control objectives, both converters
144a and 144c in certain embodiments use space vector modulation
(SVM) or other suitable modulation scheme to generate the switching
control signals 142.
[0035] Referring also to FIGS. 4 and 5, absent the disclosed grid
fault control techniques, grid faults may interrupt system power
flow and result in DC link current overshoot that could trigger the
system protection. A graph 200 in FIG. 4 illustrates preferred low
grid voltage ride through operation of the CSC-WECS 100, in which
it desired that the WECS 100 peak line-line voltage performance
operation remain above the curve 202 during and following a grid
fault condition. FIG. 5 provides a graph 210 with a curve 212
showing desired WECS reactive current control performance as a
function of grid voltage drop in order to support grid voltage
recovery without damage to the converters 110a, 110b of the
CSC-WECS 100. In the CSC 110, the instantaneous current flowing
through the switching devices S1-S12 of the converters 110a and
110b is equal to the DC link current I.sub.dc, and so long as
I.sub.dc is properly regulated, the converter overcurrent
protection apparatus will not be invoked. Moreover, during grid
fault recovery (ride-through) operation, it is desired that the CSC
110 provide proper active/reactive current to the grid 120, and
thus the DC link I.sub.dc must be maintained above a minimum
required level. The advanced (unified) DC link current regulation
techniques described herein facilitate both these goals, even where
grid fault conditions would otherwise extend beyond the DC link
current regulation control limitations of the grid-side controller
144b, since the disclosed operation selectively employs
supplemental regulation of the link current I.sub.dc using the
generator-side (CSR) controller 144a.
[0036] The low voltage ride-through requirement of FIG. 4 shows a
grid-short condition where the grid voltage drops to zero for a
time and approximately 150 ms later begins to recover along curve
202, so the WECS 100 is designed to operate above the curve 202. In
the curve 210 of FIG. 5, the WECS 100 is inputting power to the
grid 120 during fault ride-through when the voltage drops by more
than 10% of nominal, in which case the CSC-WECS 100 preferably
provides some reactive current i.sub.qs to support the grid voltage
V.sub.s, and thus to helping grid voltage recovery from a low
voltage fault. In power systems, for example, the grid operators
are actually trying to help adjust the grid frequency .omega..sub.s
and this reactive current i.sub.qs provided by the WECS 100 is
particularly helpful in grid voltage recovery because of inductive
characteristics of the grid 120 itself. That active current helps
adjust the rate of frequency .omega..sub.s which is the electrical
frequency of the grid voltage V.sub.s.
[0037] The inventors have appreciated that the DC link current
i.sub.dc in a CSC 110 can be controlled by either the grid side CSI
converter 110b and/or by the generator-side CSR converter 110a. In
certain embodiments, for normal operation with a stiff grid 120
(e.g., normal mode 310 in FIG. 7), the DC link current i.sub.dc is
conventionally controlled by the grid-side converter 110b to
achieve better performance. In the exemplary CSC-WECS 100, when a
grid short circuit fault happens, the voltage dip makes it
difficult to transfer energy to the grid 120, but during such
conditions the input power from the wind turbine-generator 116
continues to charge the DC link choke L.sub.dc if the
generator-side CSR converter 110a remains controlled according to
speed or torque regulation. In the illustrated examples, the
central regulator 144e switches to a fault mode (MODE 1 in FIG. 3)
to distribute the DC link current control task to both the
grid-side and generator-side controllers 144b and 144a which, in
turn, will manage the active power flow and maintain proper DC link
current i.sub.dc in coordinated fashion.
[0038] In this unified DC link current regulation mode, the DC link
current i.sub.dc is determined by the DC voltage difference of the
grid-side and generator-side converters (V.sub.dcr and V.sub.dci in
FIG. 1), which is stated as
v.sub.Ldc=L.sub.dcdi.sub.dc/dt=v.sub.dcr-v.sub.dci. This
relationship correlates the control of the two converters 110a and
110b in one embodiment by dividing the DC current regulator output
V*.sub.Ldc into a grid-side average DC voltage reference v*.sub.dci
and a generator-side average DC voltage reference v*.sub.dcr. The
distribution of v*.sub.Ldc upon these two parts in certain
embodiments is done according to grid voltage level, converter
ratings and operating conditions. To avoid motoring operation mode,
the lower limit of the grid side reference, v*.sub.dci,min, is set
to zero in this embodiment, although other implementations are
possible. Assuming the loss in the grid-side converter 110b is
neglected, the upper limit can be derived based on the power
calculation
V.sub.dci,max=P.sub.o,max/i.sub.dc=1.5v.sub.ds,max/i.sub.dc, where
i.sub.ds,max and P.sub.o,max are the maximum allowable active
current and power on the grid side.
[0039] The grid-side converter 110b provides the master control for
the DC link current i.sub.dc in this exemplary unified control
embodiment. If the DC link current i.sub.dc is within the grid-side
converter control capability (e.g., 0.ltoreq.-v*.sub.dci,max), the
system works in normal mode with v*.sub.dcr=0 and
v*.sub.dci=-v*.sub.Ldc in which the generator-side converter 110a
does not regulate the DC link current i.sub.dc. When the grid
voltage magnitude v.sub.ds drops and the resultant V.sub.dci,max
decreases to a level below -v*.sub.Ldc
(-v*.sub.Ldc>v.sub.dci,max), the excessive portion of v*.sub.Ldc
will be transferred to the generator-side (CSR) converter 110a to
reduce the input power from the generator 116. The average DC
voltage references are now v*.sub.dci=v.sub.dci,max and
v*.sub.dcr+v*.sub.dci.
[0040] The selection of v.sub.dci* and V.sub.dcr values is
summarized in FIG. 3. As illustrated, the unified DC link current
controller 144e includes a DC link current regulator 170 with a
V.sub.dci limit calculation component 172, a proportional-integral
(PI) controller 174, and a selection component 180 that sets the
operational mode with respect to regulation of the DC link current
i.sub.dc and provides the v*.sub.dci and v*.sub.dcr values to the
controllers 144b and 144a, respectively. In the corresponding
controllers 144b and 144a, v*.sub.dci and v*.sub.dcr are translated
to the active current references i*.sub.ds and i*.sub.qgs2 via
reference calculation components 144b2 and 144a2 of the grid-side
and generator-side converters 144b and 144a, respectively. In this
manner, neglecting losses in the converters and the DC link 150,
the reference currents used for control of the CSC 110 are
i*.sub.ds=P*.sub.o/(1.5v.sub.ds), and i*.sub.qg2=P*.sub.g/(1.5
.PSI..sub.f.OMEGA..sub.g)=v*.sub.dcri.sub.dc/(1.5
.PSI..sub.f.PSI..sub.g), where the torque current reference
i*.sub.qg of the generator-side controller 144a is the difference
between the speed regulator output i*.sub.qg1 and the calculated
torque current reference i*.sub.qg2. During grid fault periods, the
speed feedback is set to be the same as the reference, and hence
the generator speed regulator output remains the same as the
pre-fault value.
[0041] The generator speed .omega..sub.g will gradually increase
because of the reduced electrical torque because of the amount of
i*.sub.qg2 introduced from the unified controller 144e, and the
extra energy is stored as kinetic energy in the moment of inertia
of the turbine-generator mechanical system 111. This is a
reasonable situation, since the fault duration is normally very
short and if the fault lasts significantly longer, the system will
be shutdown. Moreover, a typical moment of inertia of a MW wind
turbine is around 4 to 6s, and considering the fault ride-through
requirements in E.ON grid code, the increase of the generator speed
during this period would be only around 2 to 3% even under
operation with rated wind turbine mechanical torque before the
fault happens. After the fault is cleared and the grid voltage
recovers, and v.sub.dci,max rises.
[0042] The distribution of DC link regulator output on v*.sub.dci
and v*.sub.dcr changes along with the variation of v.sub.dci,max
and the output power requirements. In response to the variation,
the grid-side controller 144b regains the control capability and
resumes to output active power with exclusive regulation of the DC
link current i.sub.dc. A rising rate limit is employed in certain
embodiments to avoid excessively rapid increases in v*.sub.dci and
helps smooth the output power. As the amount of v*.sub.dcr is
gradually retracted, the generator speed regulator of the
controller 144c again picks up the real speed feedback
.omega..sub.g and become the dominant factor to determine the
torque current, and the speed regulation starts to trace the
reference properly. All the transitions from normal operation to
fault condition or reverse are managed automatically by the unified
DC link current controller 144e in certain embodiments.
[0043] Referring also to FIG. 6, graph 220 shows maximum allowable
active current I.sub.ds,max 222 in the WECS 100, graph 230
illustrates required reactive current compensation I.sub.qs,ref
232, and graph 240 shows minimum DC link current I.sub.dc,min 242
to support grid voltage recovery in (pu) as a function of the grid
voltage V.sub.ds in the WECS 100. With respect to reactive current
compensation to support the grid voltage during grid voltage dips
and short circuit faults, the unified control 144e provides
adjustment of the DC link current reference according to the grid
side active/reactive current references. In practice, different
system operators may require different reactive power compensation
ratios. The EON code, for example, requires the generation system
to provide 2% of reactive power compensation for each 1% of voltage
dip up to a maximum of 100% of reactive current. In this case, the
reactive current will vary as shown in FIG. 6 as the grid voltage
dip level increases. In the meantime, the active power is
determined by the available wind power, the converter rating, and
also the maximum allowable current at the grid connection. Assuming
the maximum steady state operating current at the grid terminal is
1 pu, the active power output could be derived as:
i qs = { mI b ( V b - v s ) / V b ( 0.5 V b < v s .ltoreq. V b )
I b ( 0 .ltoreq. V s .ltoreq. 0.5 V b ) , and i ds . max = I b 2 -
I qs 2 , ##EQU00001##
where the base values I.sub.b and v.sub.b are the magnitudes of the
rated phase current and voltage, respectively, and the DC link
current reference can be derived as:
i*.sub.dwt=(1-w.sub.s.sup.2L.sub.oC.sub.i)i.sub.dwi-w.sub.sR.sub.oC.sub.-
1,i.sub.qs
i*.sub.qwt=w.sub.sC.sub.i(V.sub.ds+R.sub.oi.sub.ds)+(1-w.sub.3.sup.2L.su-
b.oC.sub.i)i.sub.qs
I*.sub.dc= {square root over
(i.sub.dwt.sup.2+i*.sub.qwt.sup.2)}/m.sub.ai.
[0044] The minimum operational DC link current can be obtained by
setting the modulation index of the grid-side converter, m.sub.ai
to unity. In order to provide the required reactive current, the
reference current for the unified DC link current controller 144e
should be kept above the curve 242 in the graph 240 of FIG. 6.
[0045] In accordance with further aspects of the present
disclosure, a non-transitory, tangible computer readable medium is
provided, such as a computer memory, a memory within a power
converter control system (e.g., switch control system 140 in FIG. 1
above), a CD-ROM, floppy disk, flash drive, database, server,
computer, etc.) which has computer executable instructions for
performing the above described methods. The above examples are
merely illustrative of several possible embodiments of various
aspects of the present invention, wherein equivalent alterations
and/or modifications will occur to others skilled in the art upon
reading and understanding this specification and the annexed
drawings. In particular regard to the various functions performed
by the above described components (assemblies, devices, systems,
circuits, and the like), the terms (including a reference to a
"means") used to describe such components are intended to
correspond, unless otherwise indicated, to any component, such as
hardware, processor-executed software, or combinations thereof,
which performs the specified function of the described component
(i.e., that is functionally equivalent), even though not
structurally equivalent to the disclosed structure which performs
the function in the illustrated implementations of the invention.
In addition, although a particular feature of the invention may
have been disclosed with respect to only one of several
implementations, such feature may be combined with one or more
other features of the other implementations as may be desired and
advantageous for any given or particular application. Also, to the
extent that the terms "including", "includes", "having", "has",
"with", or variants thereof are used in the detailed description
and/or in the claims, such terms are intended to be inclusive in a
manner similar to the term "comprising".
* * * * *